Home range distribution of polar bears in western Hudson Bay

Abstract

Sea ice in Hudson Bay is melting earlier and freezing later as the climate warms, resulting in declines in the condition, survival, and population size of polar bears (Ursus maritimus) in the Western Hudson Bay population. The objective of this study was to analyse temporal variation in polar bear distribution on the sea ice in Hudson Bay to determine how home range size and location may be responding to changing sea ice conditions and to examine the current population boundary. Between 1990 and 2012, 153 satellite collars were deployed on 141 adult females yielding 67,495 usable locations. We examined annual minimum convex polygons and seasonal utilization distributions. Home ranges in the 1990s (mean = 264,356 ± 30,551 km2) did not differ significantly (t16 = −1.96, P = 0.07) from those in 2004 to 2012 (mean = 353,557 ± 33,719 km2). Home range distribution of individuals differed between seasons and across years, with most variation in the freeze-up and break-up seasons. Home range size was predicted by season, ice break-up date, and individual in a multiple regression, though R 2 was low. Solitary females had smaller home ranges and were closer to land compared to females with offspring. While on the sea ice, the population boundary often encompassed only half of the 2004–2012 polar bear locations and should be reassessed. The distribution of polar bears has shifted both annually and seasonally since 2004, but the consequences remain unclear as the system is extremely variable.